Flight management systems (known by the acronym FMS) designed to prepare and thereafter automatically slave an aircraft to a flight plan are known in the prior art. The flight plan is also known by the expression “route” or “3D trajectory”. The mode of operation in which the flight management system slaves an aircraft to a flight plan is also known by the expression “managed guidance mode”. In certain situations the air traffic controller situated on the ground is required to ask the aircraft to leave its flight plan by a mode differing from the so-called “managed” mode. Managed mode is understood to mean a mode in which the aircraft is guided by the flight management system and the automatic pilot according to a three-dimensional trajectory or 3D trajectory. This happens for example when the controller gives a particular flight instruction to the aircraft for a short moment during which the aircraft no longer follows the 3D trajectory. The controller may give this instruction so as to ensure correct separation of aircraft and thus maintain a sustained rate of aircraft arrival. This flight instruction is for example an indication of speed, altitude, heading or vertical speed that must be complied with by the aircraft. During the application of the controller's instruction, the flight management system assumes, in order to carry out these various calculations, that the aircraft will return immediately to the flight plan and immediately resume tracking the scheduled vertical profile and scheduled speed setting. Thus transit time prediction calculations, performed by the flight management system, with regard to the points of the flight plan, are marred by an error proportional to the time during which the pilot performs his flight while following the air traffic controller's instruction and without rejoining the planned route and profile straight away.
In addition to these errors in the calculations of the flight management system, current flight management systems do not allow the pilot to ascertain up to what moment he can follow the air traffic controller's instructions while being able thereafter to comply with the next constraints of the flight plan. These constraints are in particular an obligation to transit a point of the flight plan, or an obligation to comply with an altitude, transit time or speed at certain points or in certain zones of the flight plan.
The notion of constraint refers to an obligation or restriction of the freedom in the manoeuvres of the aircraft that the latter absolutely must comply with. The criterion refers to a way of achieving this constraint. Within the framework of this invention the constraints are an obligation of transit in terms of position, altitude, transit time or speed with regard to certain points of the flight plan and the criteria represent the aircraft's guidance characteristics for carrying out the transits through these points.